This application claims the priority of German patent document 100 55 245.5, filed Nov. 8, 2000, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a fuel cell system as well as a method for starting a fuel cell system.
Fuel cell systems, for example, include a gas generating device that supplies hydrogen required as fuel for the operation of the fuel cell. In a reformer, for example, methanol is reformed, wherein carbon dioxide and hydrogen are generated pursuant to the following reaction:
CH2OH+H2Oxe2x86x92CO2+3H2 
By adding air/oxygen, this process can be supported through the exothermic transformation of hydrocarbon. A hydrogen-rich gas is generated through partial oxidation of the hydrocarbon.
During this reformation of methanol, carbon monoxide is also created in intermediate steps so that the reformate largely contains hydrogen, carbon dioxide, water (steam), and carbon monoxide. The approx. 1% CO concentration must be lowered to less than 40 ppm for usage of the reformate in a fuel cell because carbon monoxide drastically decreases the efficiency of polymer membrane fuel cells.
In order to remove CO in a hydrogen-rich atmosphere, a hydrogen shift reaction and selective oxidation of CO in fixed-bed reactors can be performed on appropriate selective catalysts. Such measures make it possible to reduce the CO content of the reformate to below 40 ppm.
Such methods are known, for example, from U.S. Pat. No. 5,271,916; DE 43 349 83 A1; or DE 195 44 895 C1 (U.S. Pat. No. 5,874,051).
The numerous conventional devices and methods relate to the operation of the systems once they have reached their operating temperature. When starting a fuel cell system, it is necessary to bring the CO oxidation stages as quickly as possible to the operating temperature in order to make a reformate with low CO concentrations available to the fuel cell right from the beginning.
In order to solve this problem, JP 0010029802A discloses pretreating the catalyst of the selective oxidation stage with a gas that basically consists of hydrogen at a temperature above 50xc2x0 C., wherein the gas in the catalyst reacts without the addition of air. JP 0008133701 A discloses feeding excessive oxidizing agents (air) to the selective oxidation reactor for a cold start or after interruptions whenever the temperature is below the catalyst operating temperature. This additionally oxidizes hydrogen apart from carbon monoxide, causing the temperature to increase rapidly due to the exothermic reaction. When reaching the activation temperature of the catalyst, the supply of the oxidizing agent is controlled so as to ensure optimal transformation of the carbon monoxide.
JP 04296460 A discloses the usage during the operation of a fuel cell of cooling water that was warmed up during a previous operation in order to increase the reformate gas temperature.
An object of the present invention consists of ensuring the availability of a motor vehicle driven with a fuel cell system during cold starting situations and after interruptions.
This object is resolved with a fuel cell system and a method for starting a fuel cell system according to preferred embodiments of the present invention.
Pursuant to the present invention, the cooling water stream of the fuel cell can be used to warm up the components of a fuel cell system (gas generating system, fuel cell). The cooling water of the cooling water system has a temperature of about 80xc2x0 C. downstream from the fuel cells. By storing this water in an insulating device and/or a heat-insulated container, it is possible to maintain this water temperature over longer periods of time even when the vehicle stands still. By supplying the components of the fuel cell system of the vehicle with the warm cooling water, a controlled warming of the components is possible, which in turn allows the operational readiness of the fuel cell system and/or the vehicle to be accomplished very quickly.
Pursuant to a preferred embodiment of the fuel cell system according to the present invention, a latent heat storage unit is designed within the insulating device. A latent heat storage unit may comprise, for example, of a closed body in which a suitable latent heat storage medium is located. The latent storage unit releases its stored thermal energy to the water via the body, wherein the latent storage medium, for example, solidifies. The medium melts when heating the water as soon as the water temperature has reached the melting temperature of the latent storage medium. This measure makes it possible to maintain a cooling water temperature that promotes the start of the fuel cell system over very long periods of time.
Pursuant to a particularly preferred embodiment of the fuel cell system according to the present invention, for which protection is sought separately, means for the selective supplying of heat to defined areas of the fuel cell system are provided. Specifically when cold starting a gas generating system for fuel cells, the catalytic components (e.g., reformer, CO oxidator, etc.) must be brought as quickly as possible to an operating temperature, for example, a temperature that is higher than 20xc2x0 C., so that the respective reactions occur more quickly. Particularly in exothermic stages such as CO oxidators, it has proven useful to pre-warm a small area of the catalytically active zone. When these areas are started catalytically, a reaction creates thermal energy so that the complete catalytic stage can start accordingly quickly.
It is useful if the means for the selective supplying of heat are designed as ducts that run through the defined areas. It is possible to run fine cooling water ducts through the components that need to be warmed up, comparable, for example, to the cooling water ducts of an internal combustion engine. This measure allows the components to be warmed in a controlled manner in those areas where a catalytic reaction is supposed to start.
Pursuant to another embodiment of the fuel cell system according to the present invention, a burner or a instantaneous water heater is provided for the selective heating of the cooling water. With this measure, a desired cooling water temperature can also be maintained over extended periods of time.
It is useful if the device for supplying the fuel cell with hydrogen-containing gas is designed as a gas generating unit. The catalytic components of such a gas generating system, such as reformer or CO oxidator, can be warmed up very quickly to a desired operating temperature with the measures suggested in the present invention.
It is useful if at least one catalytic element of the gas generating unit is arranged within a tubular housing. This way fine ducts can run through a portion of or the entire tube wall, with the warm cooling water being able to flow through these ducts. The production of such a tube can be realized, for example, as an extruded profile.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the present invention when considered in conjunction with the accompanying drawings.